(1) Field of the Invention
The present invention relates to electrical cables and, more particularly, to a method for producing a polyurethane protective coating about cables of the type having olefin/ester interpolymer jackets.
(2) Statement of Prior Art
In recent years there has been an increasing utitilization in marine installations of cables which employ "low-smoke" cross-linked polyolefin formulations (XLPOLYO) to meet Military Specification MIL-C-24643, "General Specification for Cable and Cord, Electrical, Low-Smoke, for Shipboard Use," dated Sep. 28, 1984. This specification which prescribed limiting standards with regard to fire retardation characteristics and production of smoke under combustion was developed by the United States Navy to improve fire safety on Navy ships and submarines.
Most polymers produce copious amounts of toxic smoke when burned; "low-smoke" XLPOLYO, on the other hand, is designed not only to generate smaller quantities of less toxic smoke when burned, but also to be somewhat self-extinguishing. Widespread use of "low-smoke" XLPOLYO-jacketed cables would allow the Navy to avoid highly destructive fires in shipboard cableways similar to that suffered by the USS TATTNAL (DDG-19) in the early 1980's. Moreover, the use of XLPOLYO jacketed cables is likely to be expanded in civilian vessels for the same reasons.
However, frequently it is necessary to splice or repair such cables, and it is necessary or desirable thereafter to provide a protective overmolding about the area of the repair or splice which will be strong and hydrolytically stable. When such activities must be performed in the field, room temperature curing of polyurethanes has been desirable since it is difficult to apply heat or radiation to facilitate curing of other encapsulants in many of the installations in which the cables are found.
Unfortunately, room temperature curing polyurethane compositions conventionally employed for such applications, were found not to bond well to the new olefin/ester interpolymer jackets. This was unacceptable for field situations where electronic cables often are spliced, attached to specialized connectors, etc. Uses such as these have traditionally required a strong, water-tight polyurethane overmold to reinforce the tensile strength of the cable and to ensure its water-tight integrity. Due to the issuance of the earlier discussed military specification (i.e., MIL-C-24643), many types of commonly used shipboard-rated electronic cables are now only available with "low-smoke" XLPOLYO jackets. Thus, the need for a successful, dependable method for bonding room temperature-curing polyurethanes to such jacket materials is becoming more pressing.
Some efforts to provide a polyurethane coating on "low-smoke" XLPOLYO jackets involved the use of heat curing polyurethanes. However, this is not desirable in the field because of the inherent requirement to use elevated temperatures for a period of time. Such a procedure requires not only a source of heat but also that the molds in which the urethane molded about the cable jacket be heat resistant, e.g., metal, and thus, an inventory of molds which would be relatively expensive and time consuming to produce. In contrast, the room temperature curing polyurethanes utilize simple conventional wax of plastic molds.
Accordingly, it is an object of the present invention to provide a novel method for producing polyurethane about electrical cable jackets of an olefin/ester interpolymer.
It is also an object to provide such a method in which the urethane molding composition may be cured at room temperature in relatively low cost molds.
Another object is to provide such a method in which the polyurethane overmolding produces a strongly bonded, water-tight covering about the electrical cable, and is thus able to reinforce the tensile strength of the cable and ensure its water-tight integrity at splices and/or repairs which have resulted in removal or opening of the underlying olefin/ester interpolymer jacket.